For example, as an apparatus for forming a thin film such as a silicon nitride (Si—N) film, a film forming apparatus has been used that performs a film forming process by airtightly loading a wafer boat (substrate holder) into a vertical reaction tube from the bottom side. A substrate such as a semiconductor wafer (hereinafter, referred to as a wafer) is then loaded in a shelf shape in the wafer boat. A gas injector for supplying a silicon-based raw material gas (e.g., a dichlorosilane (DCS) gas) and a nitrogen-based reaction gas (e.g., an ammonia (NH3) gas) to each wafer is installed inside the reaction tube. In addition, a pair of parallel electrodes for plasma-gasifying an ammonia gas are installed outside the reaction tube.
When forming the above described silicon nitride film, this film forming apparatus uses a so-called ALD (Atomic Layer Deposition) method. The ALD method is a method of performing a film formation cycle, which alternately supplies plasmas of a raw material gas and a reaction gas to each wafer multiple times. Thus, a reaction product generated by reaction of the plasmas of the raw material gas and the reaction gas is stacked on each wafer by an amount corresponding to the number of times of the film formation cycle and a thin film is formed. Accordingly, in the ALD method, the repetition number of film formation cycle correlates with the dimension of a film thickness of the thin film. In view of such a principle, it is easy to control the dimension of the film thickness.
However, in each film formation cycle, if it is intended to set a standby time until the raw material gas is completely adsorbed to the surface of the wafer, in other words, if it is intended to fulfill an ideal ALD reaction by performing an adsorption reaction until an adsorption amount of the raw material gas is saturated, a process time is lengthened, thereby decreasing throughput. That is, putting aside a theoretical reaction mechanism, when practically forming a thin film by using an actual apparatus, it is difficult to take a long adsorption time until an adsorption reaction of a raw material gas stops, so to speak, self-limitedly, in consideration of productivity, a film formation temperature of a process, and types of raw material gases in use.
Thus, when forming a thin film under a process condition that adsorption of a raw material gas is not saturated, it is very difficult to control the dimension of a film thickness of a thin film. Accordingly, when performing a film forming process on a wafer, a film formation test of a thin film is conducted also on a dummy wafer (bare wafer) for measurement of a film thickness within the same batch together with a product wafer, thereby checking the dimension of a film thickness of a thin film.
However, since a product wafer with a pattern formed on its surface has a large surface area compared to a dummy wafer without a pattern, the consumption amount of a raw material gas is increased. Thus, even though a film thickness dimension is checked using a dummy wafer, an actual product wafer easily has a smaller film thickness dimension of a thin film compared to the dummy wafer. Further, as the number of product wafers loaded in a wafer boat increases, the consumption amount of a raw material gas increases and thus it is difficult to control a film thickness dimension of a thin film even due to the input number of product wafers per a batch.
The technology of constantly maintaining an internal pressure of a buffer tank when forming an alumina film is known. However, the above-described problem has not been known.